The invention relates to a circuit in which an RF amplifier circuit also serves as a filter for a supply voltage produced from the supply voltage of the amplifier.
Usually a high frequency amplifier, primarily a radio frequency amplifier, has been implemented using a common emitter transistor i.e. the radio frequency signal to be amplified is applied to the transistor base and the amplified output signal is obtained from the collector circuit. The emitter of the transistor has in general been connected directly to the ground of the circuit. As is known in the art, the operating point of the transistor is set from the supply voltage using resistors, and the DC separation of the input and the output is performed by means of capacitors. The collector current is determined with base current, which in turn is determined by the resistance division which sets the operating point. Because of the temperature dependence of the base-emitter voltage, a temperature compensation of the bias voltage of the transistor is made, for instance, by using a diode or resistive counteraction obtained through the collector line. If the emitter has been connected to the ground of the circuit through a resistor and the emitter voltage is high, the junction voltage of the base emitter will not essentially change the collector current, whereas, on the other hand, power losses take place in the resistor connected to the emitter, through which resistor flows the emitter current approaching the magnitude of collector current runs.
The filtering of the supply voltage may be arranged in a number of ways as is known in the art. One of the basic ways is to use a simple RC filter. For such a filter to be appropriate in filtering a supply voltage line, a high ohm resistor and a high capacitance capacitor should be used. Hereby, a drawback is the great voltage loss produced by the loading current passing through the resistor and also the power loss produced herewith. A very small resistor, cannot be used because the filtering effect will be less. Another, a more profitable way is to use an emitter follower circuit in which the supply voltage to be filtered is applied to the collector of the transistor, the base of which has, through a resistor, been connected to the supply voltage to be filtered and to the ground through a capacitor. A filtered supply voltage is received from the transistor emitter. In the circuit the main current travels through the collector, and the base current, that is, the current passing through the resistor, has been reduced to a .beta. (beta) part, compared with the current passing through the simple RC filter whereby a high RC input can be used.
It has been a practice to date that the above described high frequency amplifier has been a separate circuit of its own and that the filtered supply voltage needed by other circuits has been produced using a separate circuit similar to the one described above. This has naturally added to the number of components required.